Blame Cows for Los Angeles Smog?

A study just published in the Geophysical Research Letters estimates that ammonia emissions due to dairy cattle in the Southern California air basin (SoCAB) could be almost triple the tonnage of ammonia emissions from automobiles. Ammonia serves as a key ingredient in the recipe for smog.

Are drivers off the hook, while smog fighters refocus on the Los Angeles milk suppliers?

Did Dairy Farmers Underestimate Emissions?

The new data, collected by National Oceanic and Atmospheric Administration (NOAA) aircraft on fly-overs of the SoCAB during May and June 2010, resulted in a wide range of data attributed to dairy farms. The lower end of the range -- from 33 to 176 tons of cow emissions per day -- comes in at half as much ammonia as emitted by cars, 62 tons per day. (For the statistically inclined, that is 62 ± 24 tons/day compared to 33 ± 16 to 176 ± 88 tons/day.)

The estimates for emissions from automobiles corresponds well with estimates from other emissions inventory techniques. But the ammonia data for dairies significantly exceeds results from the National Air Emissions Monitoring Study study, commissioned by the National Milk Producers Federation in cooperation with the U.S. Environmental Protection Agency. The NAEMS study calculates 60.9 grams of ammonia emitted per cow per day.

Based on the 298,000 head of cattle present in the SoCAB at the time of the fly-over study, this would amount to about 18 tons/day (which is technically inside the lower end of the fly-over range: 33-16 = 17 tons/day). The Dairyherd network notes that many dairy farms in Southern California have gone out of business. The estimate only 100,000 cows remain, responsible for 6 tons/day of ammonia.

Given how often industry-supported science arrives at different conclusions than independent studies, the new results certainly demand further study. But the large statistical uncertainties related to fly-over air monitoring leave plenty of room for industry lobbyists to fight measures to control smog by targeting dairies.

Ammonia Still is not Smog

While reducing ammonia levels can fight smog, ammonia alone does not cause smog. The ammonia must react with nitrogen oxides (Nox) in the atmosphere -- a contaminant that does not come from farms. NOx emission generators include automobiles (again) and industrial sources. So advocating public transport or electric vehicles (removing the emissions sources from the dense urban areas, or using renewable, less polluting electrical generation) remain necessary actions in the fight for clean air.